I've installed numerous electric roll drive motors over the years,
and built some pretty fancy ones, so I could not resist adding my bit
to this topic.
My first entry into electric roll drives was in the late 1970s, when
I fitted a 12V windscreen wiper motor from a Volkswagen to my pedal
player. From the outside you would never know, as a pneumatically
operated switch powered controlled the motor on-off (start pedalling
and the motor started), and the existing tempo lever moved a linear
potentiometer to vary the speed of the motor. A simple regulated 12V
power supply was all that was needed to power it.
The success of this soon saw me fitting similar motors to various other
pedal players, and even to reproducing pianos. The gearing usually
required some adaptation with the original gearing, via chain drives,
so it wasn't always straightforward. But pedalling a roll was so much
easier, and furthermore, the roll would not slow down when you pedalled
the soft bits. The motor was virtually dead silent, by the way.
That was then, but in recent months I have developed a much more
sophisticated roll drive system based around a 12V motor from a VCR.
It is essentially a standard, but nicely engineered electric motor
fitted with a tachometer, and runs at about 4000 RPM. Its speed is
directly proportional to the supply voltage (like most DC electric
motors). For an Ampico B spool, the roll drive shaft runs at around
11 RPM at a roll tempo of 80, so I had to introduce substantial
gearing, achieved by a belt drive (from the VCR), and a gear box out
of the above mentioned windscreen wiper motor.
My reason for going to all this trouble was to solve the problem of
roll acceleration. As Bill Flynt points out, if the take-up spool is
running at a constant rotational speed, the paper build-up will cause
the linear speed of the roll to increase. As a guide, a roll starting
at Tempo 80, will after about 5 minutes of playing be running at around
Tempo 90 or even higher. Certainly enough to detect, and to even make
the piece impossible to reproduce.
By using a high speed motor, it's possible to control its speed very
minutely and accurately. The electronics includes a "turns" sensor
that gives a pulse once per revolution of the drive shaft. This pulse
is counted, and the digital output of the counter is converted to
analog using a digital to analog converter (DAC0800). This analog
voltage counteracts the voltage set by the tempo control, and slows the
motor each rev of the take-up spool. By measuring the before and after
diameters of the take-up spool it's a matter of mathematics and setting
two controls to ensure a roll is held at a constant linear speed over
its entire length.
Of course not all rolls suffer from acceleration. I don't know about
Duo-Art or Welte (and would love to know), but for Ampico, rolls from
about 1926 do not have acceleration issues. By this time, Hickman
realised the all-new Ampico B would show up this problem and the
recording transport system was changed to match the characteristics
of the Ampico B spool box.
New South Wales, Australia